The present invention relates generally to matrix-addressed flat panel cathode-ray tube (CRT) displays utilizing field emission cathodes and, more particularly, to a circuit for providing brightness compensation of such a display in order to mitigate the effects of emitter irregularities which result in brightness variations.
Cathode-ray tubes are widely used in display monitors for computers, television sets, etc. to provide visual displays of information. This wide usage may be ascribed to the favorable quality of the display which is achievable with cathode-ray tubes, i.e., color, brightness, contrast, and resolution. One major feature of a CRT permitting these qualities to be achieved is the use of a luminescent phosphor coating on a transparent face. Conventional CRTs, however, have the disadvantage that they require significant physical depth, i.e., space behind the actual screen, making them large and cumbersome. There are a number of important applications in which this depth requirement is deleterious. For example, the depth available for many compact portable computer displays and operational displays precludes the use of CRTs. Thus, there has been significant interest in an effort to provide satisfactory so-called "flat panel displays" or "quasi flat panel displays" not having the depth requirement of a typical CRT, while having comparable or better display characteristics, e.g., brightness, resolution, versatility in display, power requirements, etc. These attempts, while producing flat panel displays that are useful for some applications have not produced a display that can compare to a conventional CRT.
A flat panel display arrangement is disclosed in U.S. Pat. No. 4,857,799, "Matrix-Addressed Flat Panel Display," issued Aug. 15, 1989, to Charles A. Spindt et al. This arrangement includes a matrix array of individually addressable light generating means of the cathodoluminescent type having cathodes combined with luminescing means of the CRT type which reacts to electron bombardment by emitting visible light. Each cathode is itself an array of field emission cathodes on a backing plate, and the luminescing means is provided as a phosphor coating on a transparent face plate which is closely spaced to the cathodes.
The backing plate disclosed in the Spindt et al. patent includes a large number of vertical conductive stripes which are individually addressable. Each cathode includes a multiplicity of spaced-apart, cone-shaped, electron emitting tips which project outwardly from the vertical stripes on the backing plate toward the face plate. An electrically conductive gate electrode arrangement is positioned adjacent to the tips to generate and control the electron emission. The gate electrode arrangement comprises a large number of individually addressable, horizontal stripes which are orthogonal to the cathode stripes, and which include apertures through which emitted electrons may pass. The gate electrode stripes are common to a full row of pixels extending across the front face of the backing structure, electrically isolated from the arrangement of cathode stripes. The anode is a thin film of an electrically conductive transparent material, such as indium tin oxide, which covers the interior surface of the face plate.
The matrix array of cathodes may be activated by addressing the orthogonally related cathodes and gates in a generally conventional matrix-addressing scheme. The appropriate cathodes of the display along a selected stripe, such as along one column, may be energized while the remaining cathodes are not energized. Gates of a selected stripe orthogonal to the selected cathode stripe ma also be energized while the remaining gates are not energized, with the result that the cathodes and gates of a pixel at the intersection of the selected horizontal and vertical stripes may be simultaneously energized, emitting electrons so as to provide the desired pixel display.
The Spindt et al. patent teaches that it is preferable that an entire row of pixels be simultaneously energized, rather than energization of individual pixels. According to this scheme, sequential lines are energized to provide a display frame, as opposed to sequential energization of individual pixels in a raster scan manner. This extends the duty cycle for each panel in order to provide enhanced brightness.
The present invention relates to the control of the brightness at each pixel, which is a function of the intensity of electron beam current impinging on the phosphor coating of the anode. One technique, currently in use in matrix-addressed, flat panel CRT displays, employs pulse widt modulation to control the brightness at each display pixel. This technique divides the line period into a number of intervals, wherein the time durations of each of these intervals within a single period are related according to a binary progression. Thus, for a line period comprising four intervals having time durations of one, two, four and eight time units, it is possible to provide from zero to fifteen time units of illumination at each pixel within a line period. The integrating effect of the human optic system and the retentive qualities of the phosphors on the display screen combine to translate these different-length time durations of illumination into different levels of brightness intensities.
A matrix-addressed, flat panel CRT display providing an extended range of brightnesses is disclosed in U.S. Pat. application Ser. No. 590,870, filed Oct. 1, 1990, for Peter C. Dunham, and assigned to the same assignee as the present invention. In this display the brightness control is effected by controlling both the duty cycle and the voltage applied to the drive lines of the intersecting conductors. A periodic staircase waveform having progressively increasing voltage steps is sequentially applied to the row conductors. Binary-coded video brightness data are simultaneously applied to all of the column conductors. The combined voltages at the intersections of the selected conductors cause a sequence of electron emissions onto luminescing means which results in a corresponding sequence of illumination intervals.
In one method for providing the cone-shaped tips of the electron emitting structure, a highly collimated beam of vaporized metal, illustratively molybdenum, impinges substantially normally onto a substrate, having a metal film control grid electrode with micron-sized apertures over small cavities. A second beam, illustratively aluminum oxide vapor, impinges simultaneously onto the substrate, but at a very shallow angle. During this dual deposition process, the substrate is rotated about its central axis. The net effect is that the apertures are gradually closed by the deposition of composite material (the molybdenum and aluminum oxide) while the metal cones (the cathode electrodes) are formed within the microcavities by the molybdenum vapor beam. The composite material surrounding the cones and closing the apertures is later removed by selective chemical etching.
In flat panel CRT displays of the type described above, it has been found that brightness variations across the display must be maintained to less than .+-.10% in order to provide an image of acceptable quality. A very significant contributor to brightness uniformity is the physical uniformity of the cathodes which results from the manufacturing process. Since the molybdenum cones in the above-described process tend to grow toward the source of the molybdenum vapor, the deposition beam must be highly collimated. Thus, in order to obtain uniform emitters over an area of sufficient size to be useful as a display, e.g., eight cm by eight cm or larger, the molybdenum must be evaporated from a considerable distance, typically 90 cm or more. Nevertheless, variations in the molybdenum cones resulting from typical manufacturing processes, which are often extensions of semiconductor manufacturing, are virtually unavoidable. It has been found that these variations tend to be spherical in form across the display surface.